HDD Under the Mississippi River

HDD Under the Mississippi RiverIt takes a lot for a pipeline to cross under the chief river of North America’s largest drainage system via horizontal directional drilling (HDD). To accomplish the feat, pipeline owner Boardwalk Louisiana Midstream had to navigate the waters of geology, engineering, construction machinery and state and federal agencies.

The herculean effort resulted in a successful installation of 12-in. diameter pipeline that spanned nearly 7,700 ft across the Mississippi River and two federal flood protection levees. The crossing was the keystone of a 26-mile pipeline to transport brine from a salt dome in Bayou Choctaw on the west bank of the Mississippi River to a new chlor-alkali plant on the opposite east bank of the River at Geismar, south of Baton Rouge, La.

While crossing the Mississippi River via HDD is a significant achievement, the added element of drilling under two federal flood protection levees makes this project a singular achievement. To add more complexity to the project, two HDD drill rigs jetted pilot holes from opposite sides of the river and intersected 165 ft under the river.

Engineering to Protect USACE Levees and South Louisiana
The mission of the United States Army Corps of Engineers (USACE) is to “provide vital public engineering services in peace and war to strengthen the nation’s security, energize the economy, and reduce risks from disasters.” Drilling under USACE levees is a highly regulated and prescriptive engineering and construction process. During HDD construction, the drilling fluid pressure required to remove cuttings from the drilled hole have the potential to fracture the soil, creating a pathway for drilling fluid release to the ground surface (commonly referred to as “inadvertent drilling fluid returns”). If this occurs under a regulated levee, the fissure can create the potential for the water to seep from the flood side and under the levee to the protected side during a future flood. This seepage path could lead to undermining erosion that can compromise levee stability. A project like this crossing involves establishing an early and constant line of communication with the USACE throughout the engineering, permitting, and construction processes.

As the geotechnical and HDD design engineer on the project, GeoEngineers completed a detailed desktop geology study, a thorough subsurface exploration program and designed the HDD to reduce the risk of hydraulic fracture and surface release of drilling fluid. Early in the design process, the desktop study was completed to evaluate the expected soil conditions in the area. The study identified the potential for a dense sand layer in the area of the proposed HDD alignment. The sand layer, if present, would provide a stable soil horizon in which the HDD could be designed.

To adequately characterize the subsurface soil conditions for HDD design and complete an evaluation of the hydraulic fracture potential, an extensive geotechnical exploration program was implemented. This program included drilling seven borings to depths up to 160 ft and advancing six cone penetration test (CPT) soundings to depths up to 200 ft. In general, the subsurface conditions consisted of 60 to 80 ft of soft to medium stiff clay and silt overlying medium dense to very dense sand and silty sand.

The underlying sand provided the competent soil layer to design the HDD crossing, but GeoEngineers’ HDD design team recommended the installation of casing through the weak silt and clay on both the entry and exit sides to reduce the risk of hydraulic fracture in the softer soils.

HDD Under the Mississippi RiverEngineering a Solution
The modeling of the potential for hydraulic fracture of the soil formation and drilling fluid surface release involves the representation of the soil properties, HDD contractors’ tooling and drilling fluid properties. The model results in establishing the maximum allowable drilling fluid pressure that becomes a permit requirement and must be measured during construction. In a conventional hydraulic fracture analysis, the maximum allowable drilling fluid pressure is calculated by dividing the soil formation limit pressure by a factor of safety. For this project, the formation limit pressures were divided by the required USACE factor of safety of two. This fixed factor of safety results in a variable range of allowable drilling fluid pressures across the entire HDD alignment where the allowable drilling fluid pressure varies in proportion to the soil formation limits.

Because the length of the HDD under the Mississippi River and both levees was nearly 7,700 ft, the expected drilling fluid pressure required to remove the cuttings from the drilled hole increased higher than the allowable drilling fluid pressure before reaching the exit point. As a result, the HDD installation was designed to use the pilot hole intersect method of drilling. This required the HDD to be drilled from both sides of the alignment and intersect while the factors of safety were above the USACE requirement.

Through the permitting process, the USACE reviewed the HDD design and construction plan, including the installation of the casing and using the pilot hole intersect method to reduce the risk of hydraulic fracture under the levees. The USACE required full-time documentation of the construction activities and monitoring of the downhole annular pressures.
Construction Highlights
On Jan. 15, 2013, Ranger Field Services began construction of the HDD by installing 16-in. diameter casing on both the entry and exit sides. To install the casing, Ranger jetted the pilot hole a specified distance, reamed the jetted hole, and then rotated the casing into place using the HDD drill rig. The casings were installed between 65 and 70 ft below ground surface through the soft clay and into the dense sand. The casing on the entry side was approximately 278 ft long and was approximately 259 ft long on the exit side.
Following the casing installation, pilot hole operations began from both the entry and exit sides. The observed downhole annular pressures remained below the allowable drilling fluid pressures prescribed by the USACE permit. Ranger drilled approximately 3,848 ft from the entry side and 3,830 ft from the exit side and completed the pilot hole intersect under the Mississippi River — in about 50 ft of water and about 115 ft below the mudline of the river. Ranger used two barges anchored in the Mississippi River to position secondary survey coil wires, which were used to locate the two pilot hole bits for the pilot hole intersect.

Teamwork Brings Successful Project Completion
After completing the pilot hole intersect, Ranger successfully pulled the 12-in. diameter pipe with a pre-installed PVC liner through the drilled hole, completing the first HDD installation under the Mississippi River and two USACE levees. In the end, the 7,678 ft long pipeline crossing of the Mississippi River was successful. Teamwork was key in making this challenging project happen. GeoEngineers led the HDD design engineering while other team members included: Adtech Inc. Mud Motors; DCS Fluid Solutions; GeoEngineers; Interstate Engineering; Horizontal Technology; Ranger Field Services; United Pipeline; and Vermeer Corp. All team members would like to offer a note of thanks to the professional engineering support from the New Orleans District USACE.

David P. Sauls, P.E., is senior prinicipal  and Drew Sparks, P.E., is senior geotechnical engineer at GeoEngineers.
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